A universal relationship between magnetization and local structure changes below the ferromagnetic transition in La_{1-x}Ca_xMnO_3; evidence for magnetic dimers

摘要

We present extensive X-ray Absorption Fine Structure (XAFS) measurements onLa_{1-x}Ca_xMnO_3 as a function of B-field (to 11T) and Ca concentration, x(21-45%). These results reveal local structure changes (associated with polaronformation) that depend only on the magnetization for a given sample,irrespective of whether the magnetization is achieved through a decrease intemperature or an applied magnetic field. Furthermore, the relationship betweenlocal structure and magnetization depends on the hole doping. A model isproposed in which a filamentary magnetization initially develops via theaggregation of pairs of Mn atoms involving a hole and an electron site. Thesepairs have little distortion and it is likely that they pre-form attemperatures above T_c.